This invention relates generally to tuned radio frequency (r.f.) circuits and more particularly to tunable r.f. resonant circuits.
As is known in the art, tunable resonant circuits are often used in receiver applications, such as in a radar receiver system to filter out unwanted frequency components of a signal fed thereto. In particular, bandpass filters having a narrow frequency passband are often used in r.f. receivers. One approach used in the art to provide a tunable r.f. filter is the use of a resonant circuit including a pair of coupling circuits connected to input and output ports with a ferrimagnetic body disposed therebetween. A YIG sphere is often used as the ferrimagnetic body. The principle of operation with using a YIG sphere as the ferrimagnetic material is that, in the presence of an applied D.C. magnetic field H.sub.DC, the YIG sphere of such material will provide a resonant circuit having a resonant frequency (.omega..sub.o) given as .omega..sub.o =.gamma.H.sub.DC where .gamma. is a quantity referred to as the gyromagnetic ratio. When input energy is fed to an input one of such coupling circuits, a portion of such energy having a frequency related to the resonant frequency .omega..sub.o is coupled to an output one of such coupling circuits. Conventional coupling structures for such coupling circuits are machined from a metal such as brass and electroplated with a suitable metal such as gold to reduce ohmic losses. The machined coupling circuits are then assembled into a structure to form semi-circular structures orthogonally spaced from each other at the junction of an area where a YIG sphere is disposed. This technique is costly, labor intensive, and is a difficult technique to use in fabricating such circuits. Further, the semi-circular structure disposed around the resonator body makes external access to the resonator body difficult.
When a YIG sphere is disposed between a pair of coupling circuits in the presence of a magnetic field, the YIG sphere will provide a resonant frequency related to .omega..sub.o =.gamma.H.sub.DC, at a particular temperature. An additional problem associated with such structures is that the resonant frequency of the resonator body in general is a strong function of variation in temperature and thus over the operating temperature range of such resonant circuit such resonant frequency will change. However, in certain orientations of the YIG sphere with respect to the applied d.c. magnetic field H.sub.DC, the resonant frequency of the YIG sphere is independent of variations in temperature over a wide range of operating temperatures. Since the aforementioned coupling loops nearly surround the YIG's sphere in a conventional structure, it is difficult to insert therein an "orientated" sphere, further additional orientation of the sphere is generally required to compensate for mechanical variations inherent in such coupling loop structures. Further, because of the mechanical configuration of the prior art structures, it is sometimes difficult to orientate a sphere when disposed within such coupling loops.